With a pocket saw like this you can cut through some serious logs, if you have the strength and patience to do so. Yes, a ton of effort will be required to get through a 12"  piece of wood, but at least you can have the tool available to do so if you have one of these in your emergency survival kit.

This is basically a chainsaw blade attached to handles. One cutting direction makes it easier to pull back and forth. Use two people if you can to make the job easier. Comes with a Ferro rod and striker.